Battery pack and PTC element incorporated in the battery pack

ABSTRACT

A battery pack and a PTC device are provided that can be used safely under large current loads by increasing the area of the incorporated PTC layer so as to decrease its internal resistance. The battery pack has a plurality of unit cells ( 1 ) placed side by side in parallel and the PTC device P to protect the unit cells ( 1 ) from overcurrents. The PTC device P has a first electrode ( 3 ) and a second electrode ( 4 ) connected to the both surfaces of the PTC layer ( 2 ). The PTC layer is formed to an outer shape that covers almost the entirety of the end surfaces of the two unit cells ( 1 ) to which the first electrode ( 3 ) and the second electrode ( 4 ) are connected, and has through holes ( 2 B) positioned against the end electrodes ( 1 A) of the unit cells ( 1 ). The first electrode ( 3 ) and the second electrode ( 4 ) connected to the both surfaces of the PTC layer ( 2 ) are formed to an outer shape almost the same as the outer shape of the PTC layer ( 2 ); and the first electrode ( 3 ) and the second electrode ( 4 ) are connected to the end electrodes ( 1 A) of the unit cells ( 1 ) at points where the through holes ( 2 B) of the PTC layer ( 2 ) are located.

TECHNICAL FIELD

The present invention relates to the improvement of a battery packincorporating a PTC device therein. In particular, the present inventionrelates to a battery pack and a PTC device incorporated in such batterypack wherein a plurality of unit cells placed side by side in parallelare connected in series with the PTC device.

BACKGROUND ART

PTC devices are incorporated in battery packs as protection devices. ThePTC device connected in series with unit cells will, when an overcurrentflows through the cells or a temperature of the cells rises, protect thecells by increasing its electrical resistance rapidly so that the cellcurrent is shut off or reduced to a virtually zero value.

A battery pack having a PTC device connected to unit cells in series isdescribed for example in Japanese Patent Laid-open Publication (TokkaiHei) No. 9-63553. As shown in FIG. 1, the battery pack described in thispublication has the PTC device P connected between two unit cells 1which are positioned linearly. The PTC device P has a top surfaceconnected to an end electrode 1A of one unit cell 1 and a bottom surfaceconnected to an end electrode 1A of the other unit cell. This structureis suitable for incorporating the PTC device in the battery pack wherethe unit cells are positioned linearly. However, such a PTC devicecannot connect cells in a desirable condition in a battery pack wherethe unit cells are placed side by side in parallel.

A battery pack incorporating a PTC device and having unit cells placedin parallel is described in Japanese Utility Model Laid-openPublications (Jikkai Hei) Nos. 2-69440 and 6-38157. FIG. 2 shows anexploded perspective view of the battery pack described in the formerpublication. FIGS. 3 and 4 show the PTC device incorporated in thebattery pack and the battery pack incorporating the PTC device asdescribed in the latter publication.

The battery pack shown in FIG. 2 incorporates a PTC device P having afirst electrode 3 and a second electrode 4 connected to a bottom surfaceand a top surface respectively of a rectangular PTC layer 2. The firstelectrode 3 and the second electrode 4 protrude from the both sides ofthe PTC layer 2 and are connected to the end electrodes 1A of the unitcells 1. In this structure, the battery pack has the PTC layer 2 betweenthe adjacent electrodes 1A of the cells 1 placed in parallel.

The PTC device P shown in FIG. 3 has a PTC layer 2 which is disk-shapedhaving an external size which is generally the same as the diameter ofthe unit cell and which is formed to allow the convex electrode of thecell to be inserted. The PTC layer 2 in this form can be placed aroundthe convex electrode. A plate-shaped first electrode 3 is connected tothe bottom surface of the PTC layer 2. In order to insert the convexelectrode, the first electrode 3 has a convex-shaped center portionwhich is inserted into the center hole 2A of the PTC layer 2. A second.electrode 4 having a semi-spherical section along the PTC layer 2 and aprotruding lead is connected to the top surface of the PTC layer 2. Thefirst electrode 3 is connected to the convex electrode, and the secondelectrode 4 is, as can be seen in FIG. 4, connected to the end electrode1A of the adjoining unit cell 1.

The PTC devices shown in FIGS. 2 and 3 are convenient for incorporatingin battery packs which have a plurality of unit cells 1 placed inparallel. In particular, the PTC device shown in FIG. 3 is characterizedin that it can be placed around the convex electrode so that it hardlyprotrudes from the unit cell.

However, the battery packs described in those publications are limitedby the area of the PTC layer so that it is difficult to make it larger.In the PTC device P shown in FIG. 2, the first electrode 3 and thesecond electrode 4 cannot be connected to the end electrodes 1A of theunit cells 1 if the PTC layer 2 is made larger. This PTC device P cannotbe made wider than the spacing between the end electrodes 1A of theadjoining unit cells 1.

Also, in the PTC device P shown in FIGS. 3 and 4, the outer size of thePTC layer 2 cannot be made greater than the thickness of the unit cell1. If it is made greater, it would protrude from the cell 1 so that thesize of the battery pack is locally increased. Further, the PTC layer 2in this form has the center hole 2B, which decreases the substantivearea of the PTC layer 2 that can be used.

Since the PTC device is used while connected in series with respect tothe unit cells, it is important that the internal resistance of the PTCdevice is small under normal conditions. This is because the PTC deviceconsumes power in proportion to the product of the internal resistanceof the PTC layer and the load current squared. Further, in battery packsused for large current applications, the current through the PTC devicealso increases. The PTC device can be designed to withstand the largecurrents by increasing the area of the PTC layer. In order to decreasethe internal resistance and increase the large current, it is importantto make the area of the PTC layer in the PTC device larger. However,battery packs heretofore had a shortcoming in that it was difficult toachieve this.

DISCLOSURE OF THE INVENTION

The present invention has been developed for the purpose of solving thisshortcoming. An important purpose of the present invention is to providea battery pack and a PTC device incorporated in such battery pack whichcould be used safely under large current loads by increasing the areaand decreasing the internal resistance of the PTC layer.

The battery pack according to claim 1 of the present invention has aplurality of unit cells 1 placed side by side in parallel and a PTCdevice P to protect the cells 1 from overcurrents. The PTC device P hasa first electrode 3 and a second electrode 4 attached to both surfacesof the PTC layer 2. The PTC device P is connected in series to the unitcells 1 by connecting the first electrode 3 and the second electrode 4to the end electrodes 1A of the two unit cells 1 whose end electrodes 1Aare positioned on the same or almost the same plane. It is intended thatthe term “almost” is used to include the concept of “substantially thesame plane”.

The outer shape of the PTC layer 2 is formed to cover almost theentirety of the end surfaces of the two cells 1 to which the firstelectrode 3 and the second electrode 4 are connected. Further, the PTClayer 2 has through holes 2B positioned correspondingly to positions ofthe end electrodes 1A of the cells 1.

The first electrode 3 and the second electrode 4 attached to bothsurfaces of the PTC layer 2 are formed into an outer shape that isalmost the same as the outer shape of the PTC layer 2; and the firstelectrode 3 and the second electrode 4 are connected to the endelectrodes 1A of the unit cells 1 through their portions which arelocated correspondingly to the through holes 2B of the PTC layer 2.

The PTC device according to claim 5 of the present invention protectsthe unit cells 1 from the overcurrents by connecting the first electrode3 and the second electrode 4 which are attached to the both surfaces ofthe PTC layer 2 to the end electrodes 1A of a plurality of the cells 1placed side by side in parallel.

The outer shape of the PTC layer 2 is formed to cover almost theentirety of the end surfaces of the two cells 1 to which the firstelectrode 3 and the second electrode 4 are connected. Further, the PTClayer has the through holes 2B positioned against the end electrodes 1Aof the cells 1.

The first electrode 3 and the second electrode 4 connected to bothsurfaces of the PTC layer 2 are formed into almost the same outer shapeas the outer shape of the PTC layer 2; and the first electrode 3 and thesecond electrode 4 are connected to the end electrodes 1A of the unitcells 1 at points where the through holes 2B of the PTC layer 2 arelocated.

In the battery pack according to claim 2 and the PTC device according toclaim 6 of the present invention, the second electrode 4 connected tothe top surface of the PTC layer 2 has a convex section 4A which isinserted into the through hole 2B. The second electrode 4 has the convexsection 4A connected to the end electrode 1A of the unit cell 1. It isnoted that the top and bottom directions in the present specificationare determined based on the drawings.

In the battery pack according to claim 3 and the PTC device according toclaim 7 of the present invention, the first electrode 3 connected to thebottom surface of the PTC layer 2 has a convex section 3A protrudingfrom the bottom surface at the position of the through hole 2B of thePTC layer 2. The convex section 3A is connected to the end electrode 1Aof the unit cell 1.

The battery pack according to claim 4 of the present inventionincorporates cylindrical cells as the unit cells 1, and the outer shapesof the PTC layer 2, the first electrode 3 and the second electrode 4 arerectangularly shaped with the corners bevelled to conform with thecylindrical cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art battery pack;

FIG. 2 is an exploded oblique view of another prior art battery pack;

FIG. 3 is an oblique view of a PTC device incorporated in a furtherprior art battery pack;

FIG. 4 is a top view of a battery pack incorporating the PTC deviceshown in FIG. 3;

FIG. 5 is a side view of an embodiment of a battery pack according tothe present invention;

FIG. 6 is a top view of the battery pack shown in FIG. 5;

FIG. 7 is an exploded oblique view of a PTC device incorporated in thebattery pack shown in FIG. 5;

FIG. 8 is a cross-sectional view of a PTC device incorporated in thebattery pack shown in FIG. 5; and

FIG. 9 is an exploded oblique view of another embodiment of the presentinvention.

In the drawings, the reference numbers indicate the following members:

1 . . . Unit cell,

1A . . . End electrode,

1 a . . . Flat electrode,

1 b . . . Convex electrode,

2 . . . PTC layer,

2A . . . Center hole,

2B . . . Through hole,

3 . . . First electrode,

3A . . . Convex section,

3B . . . Through hole,

4 . . . Second electrode,

4A . . . Convex section,

4B . . . Through hole,

5 . . . Casing,

5A . . . Electrode window,

P . . . PTC device

EMBODIMENTS TO CARRY OUT THE INVENTION

Embodiments of the present invention will be described below based onthe drawings. The following embodiments of a battery pack and a PTCdevice incorporated in the battery pack are given as examples embodyingthe technical concept of the present invention, and the presentinvention is not particularly restricted to the battery packs and thePTC devices described below.

Further, in order to facilitate the understanding of the claims, thepresent specification has indicated, in the “CLAIMS” and the “DISCLOSUREOF THE INVENTION”, the reference numbers corresponding to the membersshown in the embodiments. However, this by no means restricts themembers indicated in the CLAIMS to the members in the embodiments.

The battery pack shown in FIG. 5 and FIG. 6 has incorporated in a casing5 two unit cells 1 and a PTC device P for protecting the cells 1 fromovercurrent. The cells 1 are rechargeable (secondary) cells such as anickel-cadmium cell, a nickel-metal hydride cell, or a lithium ion cell.A feature of the nickel-cadmium cells or the nickel-metal hydride cellsis high current charge and discharge. The PTC device P incorporated inthe battery pack has the characteristic of increasing its electricalresistance rapidly when the overcurrent flows through the cells 1 orwhen the temperature of the cells 1 rises, thereby shutting off ordrastically decreasing the flow of current through the cells 1.

The casing 5 is a plastic molded part or a heat-shrinkable tubing andcovers the unit cells 1 and the PTC device P. The shown casing 5 haselectrode windows 5A to expose the end electrodes 1A of the cells 1. Inthe battery pack according to the present invention, it is also possibleto have a construction where, instead of exposing the end electrodesfrom electrode windows in the casing, lead wires connected to the endelectrodes are connected to external electrodes affixed to the casing.

The two unit cells 1 are placed side by side in parallel. The batterypack in the Figures has two cells 1 placed in parallel, but the batterypack according to the present invention may also incorporate three ormore unit cells. The two cells 1 are placed so that the end electrodes1A thereof are on the same or almost the same plane.

As shown in the exploded oblique view in FIG. 7, the PTC device P hasthe first electrode layer 3 on the bottom surface and the secondelectrode layer 4 on the top surface of the PTC layer 2. The firstelectrode 3 and the second electrode 4 are bonded to the top and bottomsurfaces of the PTC layer 2 so that they are connected electrically. Thefirst electrode 3, the PTC layer 2, and the second electrode 4 are, ascan be seen in the cross-sectional view in FIG. 8, intimately attachedin a laminated state of three layers.

As can be seen in the top view in FIG. 6 and the exploded oblique viewin FIG. 7, the PTC layer 2 has an external shape which is rectangularwith the corners bevelled to conform with the cylindrical cells so thatthe entire end surfaces of the two adjoining unit cells 1 are covered.The external shape of the PTC layer 2 in the Figures is slightly smallerthan the external shape of the cylindrical cells which are the cells 1.Although not shown in the drawings, the external shape of the PTC layercan also be slightly larger than the external shape of the cylindricalcells. However, if the PTC layer protrudes considerably from the cells,it would be inconvenient to place in the casing. Therefore, the size ofthe PTC layer is such that it hardly protrudes from the external shapeof the cells.

The battery pack in the Figures incorporates the cylindrical cells, butthere are also battery packs incorporating rectangular cells instead ofthe cylindrical cells. As can be seen in FIG. 9, in a battery packincorporating the rectangular cells, the PTC device P is given arectangular shape in line with the external shape of the rectangularcells to cover the end surfaces of the unit cells 1.

The PTC layer 2 has through hales 2B aligned with the end electrodes 1Aof the unit cells 1. The through hales 2B are for connecting the firstelectrode 3 and the second electrode 4 to the end electrodes 1A of thecells 1, and are made large enough to electrically connect the firstelectrode 3 and the second electrode 4 to the end electrodes 1A. In theFigures, the size of the through hole 2B in the PTC layer 2 is the sameas the external shape of the convex electrode 1 b of the cell 1. Thethrough hole, however, does not need to be the same size as the convexelectrode; and it can be larger or smaller than the convex electrode. Ifthe through holes is made smaller, the area of the PTC layer can be madelarger. However, the smaller through hole would make it difficult toconnect the first electrode and the second electrode to the endelectrodes over broad areas. Conversely, if the through hole is madelarger, the area of the PTC layer would become smaller, but the firstelectrode and the second electrode can be connected to the endelectrodes over broader areas.

The first electrode 3 and the second electrode 4 are connected to theend electrodes 1A of the cells 1 by spot welding. Therefore, the throughholes 2B are sized so that the welding electrode can be inserted intothe holes to connect the first electrode 3 and the second electrode 4 tothe end electrodes 1A. Further, in order to allow the welding electrodeto be inserted, the first electrode 3 and the second electrode 4 whichare attached to the both surfaces of the PTC layer 2 have through holes3B and 4B respectively in positions corresponding to the through holes2B in the PTC layer 2. These through holes 3B and 4B have sizes almostthe same as those of the through holes 2B. in the PTC layer 2.

The first electrode 3 and the second electrode 4 in FIG. 7 are of thinmetal sheets cut to the same external shape as that of the PTC layer 2.The first electrode 3 protrudes from the bottom surface of the PTC layer2, where the through hole 2B is positioned, to form a convex section 3A,and this convex section 3A is connected to the end electrode 1A on thecell 1. The convex section 3A has a slightly smaller external shape thanthat of the through hole 2B.

The second electrode 4 has a convex section 4A formed, which is insertedin the through hole 2B of the PTC layer 2. As this convex section 4Apenetrates the PTC layer 2 and protrudes from the bottom surfacethereof, the height of the convex section is taller than that of theconvex section of the first electrode 3. As shown in FIG 5, the firstelectrode 3 and the second electrode 4 have the bottom planes of theconvex sections 3A and 4A exactly on the same plane. With thisconfiguration, the flat electrode 1 a and the convex electrode 1 b ofthe two unit cells 1 are positioned exactly on the same plane, and theycan be connected to the first electrode 3 and the second electrode 4.

The battery pack according to the present invention, however, does notneed to position the convex sections of the first electrode and thesecond electrode exactly on the same plane. For example, the firstelectrode may be flat, or it can be shaped to protrude upwards in FIG.8, so that the protruding portion may be inserted in the through hole ofthe PTC layer. The size of the convex section inserted into the throughhole in the PTC layer is such that the convex electrode of the unit cellcan be inserted; and electrical connection is made with the convexelectrode inserted therein.

As shown in the drawings as described above, the first electrode 3 andthe second electrode 4 having the same external shape as that of the PTClayer 2 allows for a larger contact area with the PTC layer 2. The firstelectrode 3 and the second electrode 4 do not need to be the sameexternal shape as that of the PTC layer 2. For example, although notshown in the drawings, the external shape of the first electrode and thesecond electrode may be slightly smaller or slightly larger than that ofthe PTC layer.

The battery pack having the construction as described above is assembledin the following manner:

(1) The PTC device P is beforehand assembled with the first electrode 3and the second electrode 4 attached to the bottom surface and the topsurface respectively of the PTC layer 2.

(2) The PTC device P, with the first electrode 3 and the secondelectrode 4 connected, is placed on the unit cells 1 positioned side byside in parallel. The PTC device P is placed on the two cells so thatits perimeter is aligned with the perimeter of the two cells 1.

(3) A welding electrode is inserted into the through holes 2B of the PTClayer 2, and the convex sections 3A and 4A of the first electrode 3 andthe second electrode 4 respectively are spot welded and connected to theend electrodes 1A of the cells 1.

(4) The cells 1 connected to the PTC layer 2 are covered with the casing5 to form the battery pack.

INDUSTRIAL APPLICABILITY OF THE INVENTION

The battery pack and the PTC device incorporated into such battery packaccording to the present invention are characterized in that the PTCdevice has the construction facilitating the connection of the first andsecond electrodes of the PTC device to the end electrodes of the unitcells, and further that the area of the PTC layer with electrodesattached to both surfaces thereof can be made large without the PTCdevice protruding considerably from the cells. This is because thebattery pack and the PTC device according to the present invention havea unique construction for the PTC layer and the first and secondelectrodes. The PTC layer is formed in the shape that covers almost theentire end surfaces of the two cells to which the first and secondelectrodes are connected, and further the PTC layer has the throughholes positioned against the end electrodes of the unit cells. The firstand second electrodes are connected to the end electrodes of the cellsat the positions where the through holes are provided in the PTC layer.With this construction, the battery pack having the PTC layer on the endsurfaces of a number of the cells by connecting the first and secondelectrodes to the end electrodes can make the area of the PTC layerextremely large and dramatically -decrease the internal resistance ofthe PTC layer. Thus, when the unit cells are used under normalconditions, the current loss of the PTC layer is extremely small and thepower of the cells can be supplied effectively to the load. Also, byincreasing the area of the PTC layer, a large current can be passedthrough the PTC layer. Thus, the battery pack and the PTC deviceincorporated in such battery pack according to the present inventionachieve the characteristic of being able to be used safely for the largecurrent loads.

What is claimed is:
 1. A battery pack comprising a plurality of unitcells placed side by side in parallel and a PTC device for protectingthe unit cells from overcurrents connected in series to the unit cells,such connection being made by connecting a first electrode and a secondelectrode, which are connected to both surfaces of a PTC layer, to endelectrodes of two unit cells which are positioned on the same flatplane, the battery pack being characterized by: an outer shape of thePTC layer being formed to a shape covering almost the entirety of endsurfaces of the two unit cells to which the first electrode and thesecond electrode are connected, and through holes being positioned inthe PTC layer against the end electrodes of the unit cells, the firstelectrode and the second electrode connected to the PTC layer beingformed to an outer shape almost the same as the outer shape of the PTClayer, and the first electrode and the second electrode being connectedto the end electrodes of the unit cells at points where the throughholes of the PTC layer are located.
 2. The battery pack according toclaim 1 wherein the second electrode connected to a top surface of thePTC layer has a convex section which is inserted into the through holeof the PTC layer, the convex section being connected to the endelectrode of the unit cell.
 3. The battery pack according to claim 1wherein the first electrode connected to a bottom surface of the PTClayer protrudes from the bottom surface where the through hole of thePTC layer is located, and a convex section being connected to the endelectrode of the unit cell.
 4. The battery pack according to claim 2wherein the first electrode connected to a bottom surface of the PTClayer protrudes from the bottom surface where the through hole of thePTC layer is located, and a convex section being connected to the endelectrode of the unit cell.
 5. A PTC device for protecting a pluralityof unit cells from overcurrents wherein a first electrode and a secondelectrode, which are connected to both surfaces of a PTC layer, areconnected to end electrodes of the unit cells placed side by side inparallel, and the PTC device being incorporated in a battery packcharacterized by a construction wherein: an outer shape of the PTC layeris formed to a shape covering almost the entirety of end surfaces of twounit cells to which the first electrode and the second electrode areconnected, and through holes are positioned in the PTC layer against theend electrodes of the unit cells, the first electrode and the secondelectrode connected to both surfaces of the PTC layer are formed to anouter shape almost the same as the outer shape of the PTC layer, and thefirst electrode and the second electrode are connected to the endelectrodes of the unit cells at points where the through holes of thePTC layer are located.
 6. The PTC device according to claim 5 whereinthe second electrode connected to a top surface of the PTC layer has aconvex section which is inserted into the through hole of the PTC layer,and the convex section is connected to the end electrode of the unitcell.
 7. The PTC device according to claim 5 wherein the first electrodeconnected to a bottom surface of the PTC layer protrudes from the bottomsurface where the through hole of the PTC layer is located, and a convexsection is connected to the end electrode of the unit cell.
 8. The PTCdevice according to claim 6 wherein the first electrode connected to abottom surface of the PTC layer protrudes from the bottom surface wherethe through hole of the PTC layer is located, and a convex section isconnected to the end electrode of the unit cell.
 9. A battery packcomprising a plurality of unit cells placed side by side in parallel anda PTC device for protecting the unit cells from overcurrents connectedin series to the unit cells, such connection being made by connecting afirst electrode and a second electrode, which are connected to bothsurfaces of a PTC layer, to end electrodes of two unit cells which arepositioned on the same flat plane, the battery pack being characterizedby: the unit cells being rectangular cells, an outer shape of the PTClayer being formed to a shape covering almost the entirety of endsurfaces of the two unit cells to which the first electrode and thesecond electrode are connected, and through holes being positioned inthe PTC layer against the end electrodes of the unit cells, the firstelectrode and the second electrode connected to the PTC layer beingformed to an outer shape almost the same as the outer shape of the PTClayer, and the first electrode and the second electrode being connectedto the end electrodes of the unit cells at points where the throughholes of the PTC layer are located.
 10. A battery pack comprising aplurality of unit cells placed side by side in parallel and a PTC devicefor protecting the unit cells from overcurrents connected in series tothe unit cells, such connection being made by connecting a firstelectrode and a second electrode, which are connected to both surfacesof a PTC layer, to end electrodes of two unit cells which are positionedon an almost flat plane, the battery pack being characterized by: anouter shape of the PTC layer being formed to a shape covering almost theentirety of end surfaces of the two unit cells to which the firstelectrode and the second electrode are connected, and through holesbeing positioned in the PTC layer against the end electrodes of the unitcells, the first electrode and the second electrode connected to the PTClayer being formed to an outer shape almost the same as the outer shapeof the PTC layer, and the first electrode and the second electrode beingconnected to the end electrodes of the unit cells at points where thethrough holes of the PTC layer are located.
 11. The battery packaccording to claim 10 wherein the second electrode connected to a topsurface of the PTC layer has a convex section which is inserted into thethrough hole of the PTC layer, the convex section being connected to theend electrode of the unit cell.
 12. The battery pack according to claim10 wherein the first electrode connected to a bottom surface of the PTClayer protrudes from the bottom surface where the through hole of thePTC layer is located, and a convex section being connected to the endelectrode of the unit cell.
 13. A battery pack comprising a plurality ofunit cells placed side by side in parallel and a PTC device forprotecting the unit cells from overcurrents connected in series to theunit cells, such connection being made by connecting a first electrodeand a second electrode, which are connected to both surfaces of a PTClayer, to end electrodes of two unit cells which are positioned on analmost flat plane, the battery pack being characterized by: the unitcells being cylindrical cells, an outer shape of the PTC layer beingformed to a shape covering almost the entirety of end surfaces of thetwo unit cells to which the first electrode and the second electrode areconnected, and through holes being positioned in the PTC layer againstthe end electrodes of the unit cells, the first electrode and the secondelectrode connected to the PTC layer being formed to an outer shapealmost the same as the outer shape of the PTC layer, the first electrodeand the second electrode being connected to the end electrodes of theunit cells at points where the through holes of the PTC layer arelocated, and the outer shapes of the PTC layer, the first electrode andthe second electrode being rectangular with comers bevelled to conformwith the cylindrical cells.
 14. A battery pack comprising a plurality ofunit cells placed side by side in parallel and a PTC device forprotecting the unit cells from overcurrents connected in series to theunit cells, such connection being made by connecting a first electrodeand a second electrode, which are connected to both surfaces of a PTClayer, to end electrodes of two unit cells which are positioned on thesame flat plane, the battery pack being characterized by: the unit cellsbeing cylindrical cells, outer shape of the PTC layer being formed to ashape covering almost the entirety of end surfaces of the two unit cellsto which the first electrode and the second electrode are connected, andthrough holes being positioned in the PTC layer against the endelectrodes of the unit cells, the first electrode and the secondelectrode connected to the PTC layer being formed to an outer shapealmost the same as the outer shape of the PTC layer, the first electrodeand the second electrode being connected to the end electrodes of theunit cells at points where the through holes of the PTC layer arelocated, and the outer shapes of the PTC layer, the first electrode andthe second electrode being rectangular with comers bevelled to conformwith the cylindrical cells.